This PR fixes #1440 as follows:

• Removes references to the release identifier from the core specification leaving only a section labelled "last modification date" with the same requirements. Adds a note for future generations about the release identifier to explain why these requirements exist as there's no other context for them.
• Removes all reference to creating a release identifier from the reading system specification.
• Removes explanation of release identifier from the overview.
• Removes the sections on the release identifier from the multiple renditions document and moves the requirements for the unique identifier and last mod date into the publication metadata section.

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